The present invention relates to belt systems having belt alignment apparatuses and, more particularly, to belt systems utilizing relatively flexible, transparent belts and having tracking assemblies capable of compensating for lateral deviations in the position of the belt.
There are certain applications in the ink jet printing field where it is desirable to transport sheets of paper by means of a conveyor system which incorporates a belt made of a polyester film such as Mylar (a registered trademark of E. I. DuPont de Nemours & Co.). A conveyor using a Mylar belt is capable of transporting sheets of paper relatively close to the rods of a radio frequency ink dryer so that the intensity of the electric field passing through the ink particles on the paper is maximized. Another advantage of using Mylar belts is that they are relatively inexpensive when compared to belts of other materials.
A problem inherent in the use of conveyor belts made of Mylar is that such belts tend to move laterally relative to the cylindrical rollers which support them. Without constant realignment, the lateral movement of the belt after prolonged use would cause the belt to move sidewardly against supporting structure and possibly become damaged. Accordingly, with such belt systems it is necessary to provide some form of belt alignment apparatus.
Many types of belt alignment apparatuses are known. For example, the Crum U.S. Pat. No. 3,701,464 discloses an apparatus for adjusting web position in a multi-color web-fed printing press to compensate for circumferential registration error and lateral registration error. With that system, each one of a multiple printing unit system imprints its associated web with a lateral and a circumferential reference mark. A plurality of photosensors are positioned across the width of the overlapping webs so that each sensor detects a passage of a particular reference mark. The timing of the passage of the reference marks is used to calculate the circumferential and lateral error. Stepper motors are actuated to adjust printing rolls to compensate for the error.
Another example is shown in the Benson et al U.S. Pat. No. 3,627,301. That patent discloses a stripe locator which locates the position of a stripe on a web of fabric passing beneath it. Light is shone through triangular openings on a stripe sensing head and is received by photocells. The light sources and photocells are grouped in pairs on either side of the stripe so that the intensity of light received by one photocell increases as the intensity of light received by the other photocell decreases. The difference in intensity is read by a computer which actuates a tracking roller.
The Bricker, Jr. U.S. Pat. No. 3,323,699 discloses a belt-adjusting mechanism in which a moving strip of magnetic material passes beneath capacitance plates which are trapezoidally-shaped and form capacitors with the strip. The plates are positioned on either side of the strip, and the capacitance of one plate increases as the capacitance of the other decreases. This difference in capacitance is read by a controller which uses that information to actuate a belt-adjusting mechanism.
A disadvantage with devices such as those disclosed in Crum and Benson are that elaborate systems are required to detect lateral movement of a belt, which would greatly increase the overall cost of the system in which they are employed. An additional disadvantage with the Bricker, Jr. device is that it requires the use of a belt made of a metallic material.
Other devices are known in which photocells are employed in a fixed position and a beam of light is directed perpendicularly to the plane of the belt. Lateral deviation of the belt breaks the light path, causing the photocell to generate a signal which is read by a controller that actuates a tracking mechanism. Examples of that type of tracking mechanism are shown in the patents to Pages, U.S. Pat. No. 3,322,963; Cockrell, U.S. Pat. No. 2,356,567; Hlavaty, U.S. Pat. No. 2,344,817; and King, U.S. Pat. No. 2,780,669.
A disadvantage with these types of tracking mechanisms is that two separate systems are required to detect lateral movement of a belt; one for deviation in each lateral direction. Furthermore, such devices are not accurate enough to detect the relatively small-scale lateral deviations of a Mylar belt of a size used in an ink jet printing machine.
Accordingly, there is a need for a belt system having a tracking mechanism which is relatively simple in construction and economical to implement in a printing machine. Furthermore, such a tracking mechanism should be capable of use with less expensive Mylar belts. In addition, the tracking mechanism should be sufficiently sensitive to detect relatively small-scale movement of the belt in either lateral direction.